helm: Bump minimum k8s version to v1.21+

This commit is to bump minimum k8s version to v1.21. Ideally, we should
bump to v1.26 as per our support matrix, but some CI jobs are still older
versions as per below linked PR.

Relates: cilium#29888
Relates: cilium#30106
Signed-off-by: Tam Mach <tam.mach@cilium.io>

install/kubernetes/cilium/Chart.yaml

@@ -4,7 +4,7 @@ displayName: Cilium
 home: https://cilium.io/
 version: 1.16.0-dev
 appVersion: 1.16.0-dev
-kubeVersion: ">= 1.16.0-0"
+kubeVersion: ">= 1.21.0-0"
 icon: https://cdn.jsdelivr.net/gh/cilium/cilium@main/Documentation/images/logo-solo.svg
 description: eBPF-based Networking, Security, and Observability
 keywords: